Nickel is a valuable commodity and is predominantly sourced from either sulphide or laterite mineral deposits. Large high grade sulphide deposits are increasingly rare and so the processing of laterite ores is predicted to become the dominant source of the metal.
A common method of treating laterite ores is to leach the solids in acid. Acid leaching is generally followed by impurity precipitation, commonly achieved by adding limestone. Following impurity precipitation, nickel and cobalt are usually recovered from the aqueous solution together by either mixed sulphide precipitation, or mixed hydroxide precipitation. Mixed hydroxide precipitation is a relatively recent large scale industrial technology achieved by adding a basic chemical such as magnesia, lime, limestone or sodium hydroxide to the leach solution. The mixed hydroxide precipitate (MHP) consists of mostly nickel hydroxide but also contains valuable cobalt hydroxides and various other impurities. The MHP represents a more value concentrated product in that the approximately 1% nickel and 0.1% cobalt present in the original laterite ore are upgraded substantially in terms of their relative amounts in the MHP. Since the MHP has such a high valuable metal content, the feasibility of operating a centralized nickel and cobalt refinery increases. This is because the transportation costs for the upgraded intermediate product would be a fraction of that for the as-mined ore.
The MHP may be further processed in a number of ways. For example, it may be added to the melt of an iron smelter in order to alloy the contained nickel with iron. This process is not suitable for MHP with significant cobalt content as the valuable cobalt is not recovered.
Another major processing route for refining MHP is by leaching the material in an ammonia/ammonium carbonate solution. The nickel and cobalt dissolve in the ammonia solution to form ammonia complexes. Nickel is then extracted into an organic solvent to separate the nickel from the cobalt. The extracted nickel is then stripped from the organic phase and precipitated using steam. This forms a basic nickel carbonate which is then calcined to form nickel oxide which can be sold as a product in its own right or reduced using hydrogen gas to form nickel metal compacts. The cobalt is subsequently precipitated from the aqueous phase as a cobalt sulphide using hydrogen sulphide gas. This cobalt sulphide is then re-leached in acid, passed through multiple stages of solvent extraction and ion exchange to remove impurities, then switched to the aqueous ammonia system and concentrated before being precipitated as a pure cobalt oxy-hydroxide by steam stripping.
Such prior art approaches are generally either relatively energy intensive, do not return optimal nickel and/or cobalt recoveries, require an excessive number of processing stages or are sensitive to the presence of other impurities such as aluminium, iron and chromium.
There is a need for an improved method of recovering nickel from nickel containing ores. It would be desirable to provide for a straightforward separation of nickel from cobalt in MHP and enable an efficient recovery of both commodities.
Further, although the discussion above relates to the recovery of nickel and its separation from at least cobalt in a nickel and cobalt containing ore it will be appreciated that there is a need for the effective separation of a range of metals, in a similar manner, from the cobalt they are naturally associated with.